1. Field of the Invention
The present invention relates to a nucleation device for electrically inducing crystallization of a supercooled, liquid-state, heat storage medium. This invention is also concerned with a sub-assembly of such device and a method of making the same.
2. Description of the Prior Art
It is well-known in the art to use a heat storage medium of the phase change species which is adapted to utilize the latent heat absorbed and evolved during a phase change between liquid and solid states. When a crystalline heat storage medium is heated to fuse, it absorbs the heat of fusion. As the melt is thereafter allowed to crystallize, it evolves an equal amount of heat, known as the heat of solidification, which may be utilized in various heating applications.
Most typically, heat storage medium of the phase change species includes hydrate of various salts, such as NaCH.sub.3 COO.multidot.3H.sub.2 O, CaCl.sub.2 .multidot.6H.sub.2 O, Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O, Na.sub.2 CO.sub.3 .multidot.10H.sub.2 O, Na.sub.2 HPO.sub.4 .multidot.12H.sub.2 O, Ca(NO.sub.3).sub.2 .multidot.4H.sub.2 O, Na.sub.2 S.sub.2 O.sub.3 .multidot.5H.sub.2 O, KAl(SO.sub.4).sub.2 .multidot.12H.sub.2 O and NH.sub.4 Al(SO.sub.4).sub.2 12H.sub.2 O. One of the features common to these hydrates of salt as used as a heat storage medium is that they tend more or less to be supercooled. For example, When heated, trihydrate of sodium acetate (NaCH.sub.3 COO.multidot.3H.sub.2 O) begins to melt at the melting point thereof which is about 58.degree. C. However, when molten trihydrate of sodium acetate is allowed to cool, it does not freeze at 58.degree. C. but can be cooled metastably down to about -10.degree. C. to -20.degree. C. before crystallization occurs spontaneously. Such cooling of a liquid below the freezing point without solidification taking place is known as supercooling.
In certain applications, such as solar systems and heating systems powered by midnight electric, wherein storage and utilization of heat must be cyclically repeated, supercooling of the heat storage medium is undesirable because it hinders smooth heat cycling between absorption and evolution of heat. In such applications, it is desirable to interrupt or destroy the supercooled state by artificially inducing the supercooled melt to crystallize.
In other applications, to the contrary, supercooling of the heat storage medium may rather advantageously be utilized. For instance, trihydrate of sodium acetate which has a spontaneous solidification temperature of -10.degree. C. to -20.degree. C. as aforementioned may metastably remain in its supercooled liquid state at the ambient temperature. If liquid-state trihydrate of sodium acetate as supercooled down to the ambient temperature is artificially induced to crystallize at any point of time, a change of state will occur to evolve the latent heat. In the latter applications, therefore, one may control at its option the timing at which the latent heat is allowed to evolve for utilization.
Accordingly, the techniques of artificial crystallization inducement of the supercooled heat storage medium are useful in two ways.
In order to artificially induce a supercooled liquid to crystallize, it may be contacted with crystal seeds or nuclei. Otherwise, it is necessary to subject it to various nucleation treatments, such as scratching, friction, agitation, mechanical shock, extreme pressure, electric and magnetic fields, that lead to formation of crystal nuclei (J. W. Mullin, Crystallization, 3rd ed., 172 (1993); and, R. Stuart Tipson, Techniques of Organic Chemistry, Vol. 3, 414 (1950), New York).
The present inventor has previously proposed a trigger device disclosed in Japanese Utility Model Kokai Publication No. 3-96335 published Oct. 2, 1991. This device includes crystal seeds sandwiched between a leaf spring and a nut fastened thereto by a screw. By manually applying a pressure on the screw to bend the leaf spring away from the nut, supercooled liquid is contacted with the crystal nuclei to trigger crystallization. U.S. Pat. No. 4,460,546 granted to Kapralis et al discloses a mechanical nucleation trigger including a thin metallic strip having pin-hole size openings. When the trigger is snap-deformed, the edges of the openings may rub against one another to result nucleation. The disadvantage of these mechanical trigger devices is that electric or electronic control of crystallization or nucleation is difficult to achieve.
Electrical nucleation method disclosed in U.S. Pat. No. 4,529,488 granted to Kotani et al appears to have the advantage of being adapted to control nucleation by electric signals (also see Kotani et al., Journal of Japan Association of Crystal Growth 9, 31 (1982)). In this method, a supercooled melt or a supersaturated solution of salt is nucleated by application of an electric voltage. Kotani et al proposes use of electrodes of various materials including amalgamated copper which are occasionally covered by polymeric material to prevent electrode exhaustion. Kotani et al state that, while the mechanism of electrical nucleation is not clear, it is considered that an electric current changes the characteristics of the electrode surface or liquid to cause nucleation. In a later report, Kotani et al and their coauthors acknowledge that some activation treatment of electrodes was necessary in order to successfully nucleate hydrate of sodium acetate with copper-amalgam electrodes (Journal of Crystal Growth 99, 72-76 (1990)). Japanese Patent Kokai Publication No. 61-204293, published Sep. 10, 1986, also proposes to electrically nucleate supercooled trihydrate of sodium acetate.
However, the problem associated with the electrical nucleation method as taught by the prior art is that nucleation does not occur with a practical degree of reliability and repetitiveness. Thus, according to the experiments conducted by the present inventor, it has been found that some electrodes have completely failed to nucleate and that certain electrodes, while operative at the outset, have lost their nucleation function only after several tens of times of nucleation.
Accordingly, an object of the present invention is to provide an electrical nucleation device which possesses a high degree of certainty and reliability of nucleation.
Another object of the invention is to provide an electrical nucleation device which has a high degree of durability.
A still another object of the invention is to provide an electrical nucleation device which is capable of withstanding more than hundreds, preferably thousands of times of nucleation.
A further object of the invention is to provide an electrical nucleation device which is feasible for practical heat storage applications wherein storage and evolution of the latent heat are repeated for an extended period of time.
Another object of the invention is to provide a method of making an electrical nucleation device which is feasible to achieve the foregoing objects.